Field of the Invention
The present invention relates to fiber optic cables and fibers for sensing environmental conditions such as, but not limited to, hydrogen, temperature, or pressure, at one or more locations along the cables where deployed.
Discussion of the Known Art
Fiber optic cables have been used for years in the oil and gas industries for sensing temperature, pressure, seismic activity, and gas/liquid composition at various depths inside deep boreholes. For example, single mode fibers for use in distributed temperature sensing (DTS) applications are commercially available from OFS Fitel, LLC, Norcross, Ga., as type BF05717, GEO 1310 11 NA photonic fibers, and type F21976, GEOSIL®-SM single-mode fibers. See also U.S. Pat. No. 7,730,936 (Jun. 8, 2010), disclosing a DTS cable including an upper section with electrical copper conductors for producing a controlled amount of heat, and a lower section having steel conductors to produce additional heat.
In a typical temperature sensing system, a light source illuminates a proximal or near end of a sensing cable fiber. As light from the source enters the fiber and propagates away from the source, the fiber generates backscatter signals, such as Rayleigh or Raman scattering, that travel in the opposite direction back toward the source. The backscatter signals contain so-called Stokes and Anti-Stokes components that are detected and processed by commercially available measurement equipment (e.g., Silixa model XT-DTS™). Temperature values at one or more specified locations over the length of the fiber are then determined and displayed by the equipment. See, e.g., F. Suarez et al., Developments in Heat Transfer (InTech 2011), at pages 611-36; and G. Brown, Downhole Temperatures from Optical Fiber, Oilfield Review, v. 20, no. 4 (2009), at pages 34-39.
Accordingly, an optical fiber contained in a temperature sensing cable must perform two functions, namely, (i) generate the backscatter signals in response to the light signals originating from the light source, and (ii) transmit or return the backscatter signals with enough strength so that the measurement equipment can detect the signals adequately to yield reliable temperature measurements. See, e.g., TU Delft (NL), Distributed Temperature Sensing (2015), at www.citg.tudelft.nl/?id=20298&L=1.
For details on incorporating multiple environmental sensing functionalities within a single fiber structure or “multicore” fiber, see X. Sun et al., “A Multicore Optical Fiber For Distributed Sensing”, Proc. SPIE 9098, Fiber Optic Sensors and Applications XI, 9098W (Jun. 18, 2014), published online in PDF format at www.ofsoptics.com/oil-gas-distributed-temperature-sensing.html, which is incorporated by reference. See also, U.S. Pat. No. 8,737,792 (May 27, 2014) and U.S. Pat. No. 8,725,001 (May 13, 2014); and P. S. Westbrook et al., “Integrated optical fiber shape sensor modules based on twisted multicore fiber grating arrays,” Photonics West (July 2014), all of which are also incorporated by reference.
Problems arise, however, due to the fact that after most sensing fibers are exposed to so-called darkening agents in a sensed environment, the amount of attenuation that the fibers introduce to light signals propagating in them increases substantially and rapidly. Such darkening agents include, for example, high hydrogen partial pressure and ionizing radiation, one or both of which usually exist in deep boreholes and other sensed environments. High temperature (e.g., around 350 deg. C.) in sensed environments is known to accelerate hydrogen darkening, and some temperature increases may accelerate radiation darkening. See, T. Geisler et al., “Radiation performance of low bend-loss optical fiber for gyroscope applications,” Position, Location, and Navigation Symposium (May 2014).
Also, most sensing fibers have cores that are doped with germanium (Ge) which makes them particularly susceptible to darkening in reaction to hydrogen. That is, it has been found that the germanium in the fiber core reacts with hydrogen in the environment to produce OH functional groups which, in turn, significantly increase fiber attenuation. See U.S. Pat. No. 8,401,355 (Mar. 19, 2013), incorporated by reference. FIG. 3 of the '355 patent and related text disclose that at 150 deg. C. and one atmosphere H2 pressure, the propagation of light signals through a pure silica core fiber was substantially unaffected. By contrast, the attenuation induced in a sensing fiber with a Ge doped core under the same conditions was as much as 10 dB/km higher than that induced in the pure silica core fiber.
See, OFS, “Producing Oil and Gas,” at www.SpecialtyPhotonics.com, and which is incorporated by reference. The hydrogen performance of fibers with pure silica and Ge doped cores was tested after the fibers were exposed to 100 psi pure hydrogen at 280 degrees C. for 350 hours. Results showed that for a multimode fiber having a pure silica core (OFS product GeoSil-MM), attenuation increased by less than 1 dB/km over the Raman band from 1014 to 1114 nm. By contrast, a multimode fiber with a Ge doped core showed an increase in attenuation of 300 dB/km at 1550 nm, and of 100 dB/km at 1060 nm. Further, the attenuation in a single mode fiber with a pure silica core (OFS product GeoSil-SM) increased by not more than 0.05 dB/km at 1550 nm under the same test conditions.
With respect to the effects of radiation on optical fibers including fibers doped with Ge, see E. J. Friebele et al., Development of Radiation-Hard Fiber for IFOGs, Optical Fiber Sensors, OSA Technical Digest (CD), Optical Society of America (2006), at www.opticsinfobase.org/abstract.cfm?URI=OFS-2006-ME2, which is also incorporated by reference.
It will therefore be understood that when darkening agents in the environment act to increase the attenuation of light signals traveling in a sensor fiber, the length of the fiber that remains useful for transmitting signals representing a sensed condition to measurement equipment many kilometers from where the signals originate in the fiber, decreases. Accordingly, there is a need for a distributed environmental sensing optical fiber or cable whose useful length is not adversely affected by darkening agents in the environments where the cable is deployed.